why don't you just look in the mirror, think about something happy or a person you really like (in a platonic way), look at your face, then try to replicate that. Smiling is NOT just curving your mouth (if you do that you'll probably just look creepy and fake). It's all about the eyes, though it sounds like you've already got that part down pat.
And/or, see others who are doing a good job of being friendly-looking, like your boss, memorize their expression, replicate it in a mirror, pay attention to the muscles you're using.
Alternatively (or in combination), find something to like about the strangers you're talking to, or at least try to see strangers positively instead of as a nuisance. It's hard to fake a good smile if you're talking to people you despise, although it can be done with practice.